The dramatic rise in output power from rare-earth-doped fiber sources over the past decade led to a range of fiber-laser system with outstanding performance in terms of output power, beam quality, overall efficiency, compactness and wavelength flexibility. In general, the upsurge in output power became possible due to several factors including, among others, development of large mode diameter double clad (“LMADC”) fibers and the increase in the power and brightness of semiconductor diode pumps briefly discussed below.
The development of active DC fibers is associated with increasing core size which lowers susceptibility to nonlinear effects (“NLE”) due to a large mode area, improves the core-to-cladding area ratio and enhances pump absorption. However, the large-diameter cores supporting substantially only a SM also impose significant constrains in step-index fibers. As known to one of ordinary skill in the fiber laser art, a SM fiber is characterized by normalized frequency parameter V=Rco (2π/λ)√{square root over (Δn)} equal to bout 2.405, where Rco−core radius, Δn=ncore2−nclad2. To prevent generation of high order modes in DC fibers, thus, a NA of fiber should be considerably reduced which leads to weak guiding in the core causing, in turn, the fiber to be overly sensitive to bending losses.
The development of modern laser diode pumps also mightily contributes to the continuous power increase and brightness particularly when diode pumps are used in combination with LMADC fibers. However, the level of pump power is not limitless due to many factors including the known geometries of fiber amplifiers which limit the scalability of fiber devices, as discussed below.
Referring to the fiber systems including a pump/DC configuration, pump absorption depends on the core/clad diameter ratio of amplifying fibers. The core size cannot be enlarged above a certain value because of the excitation and amplification of high order modes (“HOMs”) which, if excited, immediately decrease the quality of the output light below acceptable levels. The limited core dimensions also restrain the size of the clad since, otherwise, the core/clad diameter ratio would be unacceptably reduced which, in turn, would decrease the brightness and absorption of pump light. As a consequence, the limitation in the clad and core diameters affects the amount of pump light which, in turn, limits power of fiber amplifiers.
Recently, the fiber laser industry has turned to crystal fiber rods typically used in output stages of amplifier chains to address the scalability of fiber amplifiers. Based on airhole clad technology, a crystal fiber rod includes a double clad structure with an inner cladding defining an increased diameter pump core.
The doped core of the fiber rod has a very small NA, and is made from fused silica/quartz which typically hosts a low dopant concentration. The above-mentioned characteristics may detrimentally affect the scalability of a laser system based on a fiber rod as explained below.
A low numerical aperture, which typically does not exceed about 0.02, limits the amount of high power pump light which may be coupled into the core in amounts necessary for reaching ultrahigh powers in a kW-MW range depending on whether a laser system operates in CW or pulsed regimes. Accordingly, a sufficient amount of pump light can be coupled only into the pump core/inner cladding. One of ordinary skill in the fiber laser arts knows that absorption of clad-coupled pump light is about 8 times less than that of core-coupled light. To fully utilize clad-coupled pump light, thus, a fiber rod should have, a length varying between several tens (typically exceeding 50) of centimeters and meters. Even the shortest available fiber rod thus is detrimentally affected by the presence of NLEs. The latter, of course, critically limits the laser's power scalability.
A low concentration of ions, such as ytterbium (“Yb”), is typically about 700 ppm. With such a low dopant concentration, absorption of pump light is also low. For example, in certain applications the use of a Nd-doped fiber laser pump at 920 nm is highly desirable. However absorption at 920 nm by a small concentration of Yb dopants is particularly low. To provide adequate absorption, the overall length of a fiber rod should be increased. As discussed above, increasing the overall length lowers a threshold for NLEs which, in turn, limits the amplifier's power scalability.
An open-end structure of fiber rods is also problematic. Typically, launching an input signal throughout air gaps can be realized only by micro-optics. The latter, of course, complicates the overall laser configuration making it cumbersome and expensive. The presence of air in gaps or holes lowers thermal conductivity properties. In particular, the air holes slow dissipation of heat which, in turn, may damage the rod itself and cause an environmental hazard.
The known high power fiber laser systems are far from being compact which is highly desirable due to typically limited space in which these systems are deployed. In addition to substantial lengths of fiber waveguides, as a rule, the known systems include a long delivery passive fiber guiding the amplified signal light to the workpiece or surface to be laser treated. Subject to the local requirements, the length of a delivery cable is practically unlimited and can reach tens of meters and more. Besides the compactness, the use of passive fiber delivery cables also affects power losses. A delivery cable should be spliced directly or indirectly to the output of the amplifier fiber which may result in splice power losses of signal light. Even low power losses are undesirable in a highly competitive world of high power fiber laser systems.
A need therefore exists for an amplifying fiber configured to absorb more than about 90% of multi-kW multimode (“MM”) pump light along a relatively short fiber length and emit a multi-kW output in substantially a fundamental mode.
Another need exists for the above-described amplifying fiber configured with a NA considerably higher than that of regular LMADC fibers.
Still another need exists for an ultra-high power fiber laser system with the described above amplifier fiber delivering amplified pump light directly to a laser head.
A further need exists for the ultra-high power fiber laser system configured with a multi-kW pump system emitting pump light which is controllably coupled into the desired region of the inner cladding of the amplifying fiber.